tail: fix race between read and fstat

tail: fix race between read and fstat

* src/tail.c (get_file_status): Remove, since after the changes
described below it would be called in just one place and it’s a
bit clearer to inline by hand.
(tail_file): Don’t call fstat after reading the file, as that
misses changes arriving between read and fstat.  Instead, reuse
the fstat done before reading the file.